foo.js

module.exports = function(cb) {
	let state = 'idle', cancel = null, pending = null;

	function submit(promise) {
		let cancelled = false;
		promise.then((res) => {
			if (!cancelled) {
				cb(res);
				oncomplete();
			}
		});
		return () => { cancelled = true; };
	}

	function oncomplete() {
		switch (state) {
			case 'idle':
				console.error("oncomplete() called in idle state, this should never happen");
				break;
			case 'working':
				cancel = null;
				state = 'idle';
				break;
			case 'pending':
				cancel = submit(pending);
				pending = null;
				state = 'working';
				break;
		}
	}
	
	return {
		run(promise) {
			switch (state) {
				case 'idle':
					cancel = submit(promise);
					state = 'working';
					break;
				case 'working':
				case 'pending':
					pending = promise;
					state = 'pending';
					break;
			}
		},
		clear() {
			if (cancel) {
				cancel();
				cancel = null;
			}
			pending = null;
			state = 'idle';
		}
	};
}